Status of wheat rust research and control in China Zhensheng Kang*, Jie Zhao, Dejun Han, Hongchang Zhang, Xiaojie Wang, Chenfang Wang, Qingmei Han, Jun Guo & Lili Huang *College of Plant Protection and Shaanxi Key Laboratory of Molecular Biology for Agriculture, Northwest A&F University, Yangling, Shaanxi 712100, P. R. China e-mail: [email protected] Abstract In China, wheat is grown on approximately 24 million hectares with an annual yield of 100 million tonnes. Stem rust, caused by Puccinia graminis f. sp. tritici, is a threat mainly to spring wheat in northeastern China. Leaf rust, caused by P. triticina, occurs on crops in the late growth stages in the Yellow-Huai-Hai River regions. Stripe rust, caused by P. striiformis f. sp. tritici (Pst), is destructive in all winter wheat regions and is considered the most important disease of wheat in China. During the last 20 years, widespread stripe rust epidemics occurred in 2002, 2003, and 2009, and localized epidemics occurred in many other years. In recent years, major yield losses were prevented by widespread and timely applications of fungicides based on accurate monitoring and prediction of disease epidemics. A total of 68 Pst races or pathotypes have been identified using a set of 19 differential wheat genotypes. At present, races CYR32 and CYR33 virulent to resistance genes Yr9, Yr3b, Yr4b, YrSu and some other resistance genes are predominant. Moreover, these races are virulent on many cultivars grown in recent years. Of 501 recent cultivars and breeding lines 71.9% were susceptible, 7.0% had effective all-stage resistance, mostly Yr26 (= Yr24), and 21.2% had adult-plant resistance. Several resistance genes, including Yr5, Yr10, Yr15, Yr24/Yr26, YrZH84 and some unnamed genes, are still effective against the current Pst population. All have been widely used in breeding programs. Lines with one or more of Yr1, Yr2, Yr3, Yr4, Yr6, Yr7, Yr8, Yr9 and other unnamed resistance genes are susceptible to currently predominant races. Durable adult plant resistance sources are being increasingly used as parents in breeding programs. Progress has been made in genomics and population genetics of Pst, molecular mapping of resistance genes, and cytological and molecular mechanisms of the host-pathogen interactions involved in stripe rust. Keywords disease management, epidemiology, leaf rust, stem rust, stripe rust, wheat Introduction Wheat (Triticum aestivum L.), the second largest food crop in China, was grown on approximately 24 million hectares in 2009 producing more than 100 million tonnes of grain. Barley (Hordeum vulgare L.) is annually grown on about 1 million hectares in the Yangze River valley, Huanghe River valley and Qinghai-Tibet Plateau where leaf rust and stripe rust are now the most important diseases of barley. Stripe rust is especially destructive on barley in Tibet (Wang ZH et al. 1989; Niu et al. 1994). Oats (Avena sativa L.) are planted on roughly 1.2 million hectares in Inner Mongolia, Hebei, Shanxi, Gansu, Shaanxi, Yunnan, Sichuan, Ningxia, Guizhou and Qinghai. Stem rust and crown rust on oats occur in the oat-growing areas, and are the most important diseases in some regions. Oat stem rust in 50 BGRI 2010 Technical Workshop, 30-31- May 2010, St Petersburg, Russia Kang et al. Status of wheat rust research and control in China particular, can cause severe epidemics in Inner Mongolia and northeastern China in some years (Yang 1984). Leaf rust (brown rust), caused by Puccinia triticina Eriks., was earlier a serious disease of wheat in southwestern China and the Yangze River valley. In the last three decades, it has become more important in northern, northwestern and northeastern China. Significant yield losses were caused by leaf rust in northern China in the 1970s (Zhang and Liu 2001). Research on leaf rust of wheat is mainly conducted at Hebei Agricultural University, Baoding, Hebei. Stem rust (black rust), caused by P. graminis Pers. f. sp. tritici Eriks. & Henn., causes severe yield reductions in some years in northeastern China and the Yellow-Huai-Hai River areas (He et al. 2008). Currently, investigations on wheat stem rust are mainly carried out at Shenyang Agricultural University, Shenyang, Liaoning. The spread of race Ug99 (TTKSK) with Sr31 virulence from Africa to Asia is a potential threat to wheat production in China, but the current risk level is considered to be low. Of 700 Chinese major wheat cultivars and breeding lines tested in Kenya, only four were highly resistant, 10 were moderately resistant, and the remaining 686 genotypes were highly susceptible (He et al. 2008). Therefore, if race Ug99 were to appear in China severe stem rust epidemics could follow. Stripe rust (yellow rust), caused by P. striiformis Westend. f. sp. tritici Eriks., is the most destructive disease in all winter wheat growing regions in northwestern, southwestern, and northern China, as well as in the spring wheat areas in the northwest (Li and Shang 1989; Wan et al. 2004; Chen WQ et al. 2009). Severe epidemics in 1950, 1964, 1990, and 2002 resulted in yield losses of up to 6.0, 3.2, 1.8, and 1.3 million tonnes, respectively (Wan et al. 2007; Chen WQ et al. 2009). Stripe rust is much more important than leaf rust or stem rust based on historical data and current disease situations. Plant pathologists, breeders, farmers, and governmental organizations have given much attention to research on, and control of, stripe rust. Nationwide cooperation on various aspects of the disease has occurred since 1950, mainly focusing on epidemiology, race identification, breeding resistant cultivars, and integrated management. In this paper, we will summarize recent research on, and control of, wheat stripe rust in China, including occurrence and control, evolution of virulence, and molecular research. Also, challenges and strategies for controlling stripe rust in China will be discussed. Epidemiology, occurrence and control of wheat stripe rust in recent years China is considered the largest independent epidemic region in the world (Stubbs 1985). Based on historical epidemiological data for stripe rust, the wheat-growing regions can be divided into the western over-summering areas, the over-wintering areas, and the eastern epidemic areas (Li and Zeng 2002). The over-summering areas include the northwestern (Shaanxi, Gansu, Sichuan, Ningxia, and Qinghai), southwestern (Yunnan and Guizhou), and Xinjiang regions (Li and Shang 1989; Li and Zeng 2002). Stripe rust can complete its year-round cycle in Xinjiang, Yunnan, southern Gansu and northwestern Sichuan where wheat can be grown from lowland valleys at 1,000 m to highland terraces at 3,300 m which provide a “green bridge” for pathogen migration from late-maturing highland areas to early-sown wheat plants in the lowlands (Li and Zeng 2002; Zeng and Luo 2006; Chen WQ et al. 2009). The over-wintering regions are mainly in Sichuan, Hubei and Shaanxi. The eastern epidemic areas cover the largest wheat-producing areas, including Henan, Hubei, Shandong, 51 BGRI 2010 Technical Workshop, 30-31- May 2010, St Petersburg, Russia Kang et al. Status of wheat rust research and control in China Shanxi, Hebei, Sichuan, Shaanxi and Anhui. In over-summering areas, the infected autumn-sown wheat and volunteer wheat in the lowlands, and late-maturing spring wheat in the highlands serve as inoculum sources for local recycling throughout the year (Li and Zeng 2000). Previous studies showed that urediniospores from the over-summering regions, particularly the northwestern and southwestern areas, spread eastward to eastern China. Fall-sown winter wheat in the over-wintering regions becomes infected during late autumn and early winter and the pathogen survives mainly as latent mycelial infections until temperatures increase in spring when inoculum then moves to the major winter wheat regions to the northeast (Li and Zeng 2002; Zeng and Luo 2006). Thus, interregional disease distribution in time and urediniospore spread are mainly from west to east in autumn and from south to north in spring (Li and Zeng 2002; Zeng and Luo 2006). The over-summering areas provide initial inoculum for the eastern plain regions of China (Li and Zeng 2002; Zeng and Luo 2006). Over the last decade wheat stripe rust has remained at high levels posing a threat to wheat production across the entire country. This follows the development and spread of races CYR32 detected in 1994 and CYR33 found in 1997 which are now the predominant races (Chen WQ et al. 2009). The main reason is that winter wheat cultivars with Yr9, Yr3b, Yr4b and YrSu, now susceptible to both races, account for 90% of total area of winter in the entire country. Survey data from the last ten years show that the areas affected annually by stripe rust (Table 1) were on average about 4 million hectares. For example, stripe rust affected 6.6, 4.9, and 4.08 million hectares in 2002, 2003, and 2009, respectively (Table 1). It is considered that the stripe rust epidemic in 2002 was the most widespread in the past three decades (Li and Zeng 2002; Wan et al. 2004). TABLE 1 HERE In 2009, the early occurrence of stripe rust, 10-55 days in earlier than usual, posed the greatest threat for many years, following widespread over-wintering of urediniospores due to mild winter and favorable early spring conditions. It was estimated that the total area of wheat infected by stripe rust reached 4.08 million hectares (Table 1). Fortunately, intense disease monitoring and forecasting allowed timely application of fungicides which effectively prevented losses and further spread to the wheat production regions further east. Thus, a potentially huge yield loss nationwide was avoided through timely use of fungicides based on earlier accurate disease forecasts. Moreover, the surveys and forecasting of disease provided information to pathologists, growers, county agents, extension services, the fungicide trade, and government administrators from village level to the Ministry of Agriculture enabling them to make decisions on chemical intervention to minimize yield losses.